1. Field of the Invention
The present invention relates to an engine control system and an engine control method. Particularly, the present invention relates to a technology to control an engine that is equipped with a catalyst to purify exhaust gas.
2. Description of the Related Art
A hybrid vehicle equipped with a motor as a drive source in addition to an engine is generally known. In the hybrid vehicle, the engine load may be reduced with the assistance of the motor.
As with a non-hybrid vehicle that is equipped with only an engine as the drive source, the engine of the hybrid vehicle also discharges exhaust gas. Thus, it is desirable to operate the engine such that an amount of the exhaust gas is not excessive.
Japanese Patent Application Publication 2001-37008 (JP-A-2001-37008) describes that, when electricity is generated to obtain desired electric energy, the engine operation range is determined based on a discharge amount limit of exhaust emissions such as nitrogen oxide and smoke or based on a parameter that significantly affects exhaust performance. Then, the engine speed is corrected to operate the engine within the above range, and electricity is generated.
According to the technology described in JP-A-2001-37008, the increase of the exhaust emissions is reduced to a minimum level when electricity is generated. Further, the electricity generated is utilized to compensate for output when the output is required such that the discharge amount limit of the exhaust emissions may exceed. Accordingly, it is possible to reduce the amount of the exhaust gas and to improve fuel economy under various driving conditions.
The exhaust gas discharged from the engine is purified by a catalyst. The purification efficiency of the catalyst may vary in accordance with the temperature of the catalyst. For example, the catalyst cannot purify the exhaust gas until the temperature of the catalyst is equal to or above a prescribed temperature. However, if the temperature of the catalyst becomes too high, the purification efficiency of the catalyst deteriorates. Thus, as in the technology that is described in JP-A-2001-37008, even if the engine operation range is determined based on the discharge amount limit of the exhaust emissions or based on a parameter that significantly affects the exhaust performance, and even if the engine speed is corrected to operate the engine within the above range to generate electricity, the catalyst temperature may be too high to allow effective purification of the exhaust gas by the catalyst.